The patent EP-B-0761493 describes a modular power supply line in which each module comprises an elongated casing made of insulating material defining an inner cavity which extends in a rectilinear direction. The insulating casing, in use, is adapted to be embedded in a road surface with an upper portion thereof flush with said road surface. The inner cavity houses a flexible belt element provided with portions made of ferromagnetic material adapted to interact with the magnetic field generated by a vehicle (for example a railway vehicle) which moves along the power supply line.
The power supply line furthermore comprises a plurality of flat conducting elements borne by the upper portion of the casing, aligned in a rectilinear direction and substantially flush with the road surface.
In the absence of magnetic attraction, the belt element is arranged, throughout the length of the module, in an idle position in which it is substantially rectilinear and not deformed and the conducting elements are not powered and/or are connected to a negative reference potential (ground). The power supply line therefore automatically goes to a state of insulation, in the absence of magnetic activation.
In the presence of magnetic attraction originating from the vehicle that passes over the casing, the portion of belt element affected by the magnetic field bends upwards, deforming roughly in the shape of a sine wave; said raised portion of the belt element arranged in an activation position creates an electrical bridge connection between a positive power supply line inside the casing and at least one conducting element which is connected to the positive power supply line. The conducting elements arranged at the sides of the element are connected to the reference potential (ground).
The movement of the vehicle with respect to the casing moves the raised portion of the belt element along the inner cavity, allowing sequential power supply of the conducting elements.
The electric vehicle is provided with a collector device which creates the above-mentioned magnetic attraction and allows the live conducting element to be connected to an electrode which provides the power supply for the electric vehicle.
The above-mentioned self-insulated power supply line has a series of drawbacks including:
a) the problems connected with ensuring the safety of the flat conducting elements and therefore maintenance of the above-mentioned state of insulation;
b) use of the line in disturbed external ambient conditions which can induce surface dispersion currents that spread over the road surface from the live flat conducting elements;
c) safe management of the conducting elements in the event of malfunctioning or breakdown of the line.
In relation to point b) illustrated above, during normal operation, the self-insulated lines of the above-mentioned type are not able to provide and above all maintain over time a high level of insulation between the conducting elements powered and those connected to the reference potential for a plurality of reasons including:                intrinsic operation of the above-mentioned line in which the conducting elements are connected in sequence to the positive power supply line and to the earth line—for this reason the physical distance between conducting elements with opposite polarities is limited to allow power supply continuity to the moving vehicle;        the conducting elements are positioned and installed on the road surface which guarantees a sufficient level of insulation only in certain operating conditions (perfectly dry and completely clean road surface).        
In the presence of dampness, dirt or water on the road surface the insulation level drops to very low values.
Due to said poor insulation, a dispersion current is present—in almost all operating conditions of the line—which flows between the live conducting elements and those connected to the reference potential.
The main problems connected with the presence of this dispersion current are the following:                the dispersion current cannot be discriminated from the traction current absorbed by the vehicle;        the dispersion current is incorporated in the measurement of the overall current delivered; and        the dispersion current could generate dangerous potentials on the road surface.        
The patent WO 98/36933 describes a power supply line of the type described in the patent EP-B-0761493 and provided with a series of sensors (switches) which are closed by the belt element arranged in the idle position and opened by the belt element arranged in the activation position. In particular the sensors comprise conductive pads electrically spaced from one another and borne by the casing and interconnection means borne by the belt element and adapted to provide an electrical bridge connection between the conductive pads for the portions of the belt element arranged in the idle position.
According to a variation described at the end of the document WO 98/36933, a resistance measuring device is used adapted to measure the resistance between conductive pads and metallic portions of the casing to detect, in the case of low resistance, the presence of conducting agents (such as water and steam) inside the casing.
The document WO 98/36933 does not provide only indications on the insulation inside the casing and cannot provide any indication of, the dispersion condition outside the casing. In particular, the solution described at the end of the document WO 98/36933 is not able to detect the surface dispersion currents that spread over the road surface from the live flat conducting elements when the road surface is partially conducting.